1. Field of the Invention
The present invention generally relates to methods for deciding when and how a plurality of users or nodes can access a wireless medium, data structures supporting such methods and devices that are specially adapted for using and supporting such methods.
2. Description of the Related Technology
In a wireless communication system the wireless medium is shared by all the users or nodes in the system. It is a big challenge to schedule the radio resource among multiple users to avoid collisions resulting from two nodes sending data packets at the same time over the same channel, and also to restrict multiple access interference. Hence, medium access control (MAC) protocols are developed to provide such coordination. In the seven-layer OSI protocol stack, the MAC layer is normally specified as a sublayer of the data link layer (layer 2).
MAC protocols have been extensively studied in traditional wireless communication systems. They can be roughly divided into two categories: contention-based and schedule-based.
Time division multiple access (TDMA), frequency division multiple access (FDMA) and code division multiple access (CDMA) are widely used techniques in modern cellular communication systems. Their basic idea is to explicitly assign transmission and reception opportunities to nodes and let them sleep the rest of the time. The radio resource, or channel, is differentiated by time, frequency or orthogonal codes. Since these sub-channels are orthogonal to each other, MAC protocols in this category are collision-free, and hence no special mechanisms are needed to avoid hidden-terminal problems. They are referred to as schedule-based protocols. There are also drawbacks related to schedule-based MAC schemes. Since all channels are allocated by a unique central controller, additional signaling traffic is needed to setup and maintain the schedules. For example, if TDMA is employed, all nodes in the network have to be synchronized and agree on the slot boundary. Otherwise, timeslot overlap also leads to collision. Hence, regular signaling for synchronization is a fixed overhead in such networks. Another drawback is that it is not easy to adapt the schedules to different traffic loads on small timescales.
Another class of MAC protocols is based on contention. Rather than using pre-allocated channels, nodes contend on-demand for a shared channel, resulting in probabilistic coordination. Collision may occur during the contention procedure in such systems. Classical examples of contention-based MAC protocols include ALOHA and carrier sense multiple access (CSMA). In ALOHA a node simply transmits a packet when it is generated (pure ALOHA) or at the beginning of the next available slot (slotted ALOHA, whereby time is partitioned into equally-sized time slots, each time slot being big enough to accommodate a packet). Packets that collide are discarded and retransmitted later. In CSMA a node listens to the channel before transmitting. If it detects a busy channel, it delays the access attempt and retries later. Otherwise, it begins data transmission immediately in the following timeslot or begins data transmission after the exchange of channel access confirmation. The CSMA protocol has been widely studied and extended. Today it is the basis of several widely-used standards including IEEE 802.11 and IEEE 802.15.4. The contention-based MAC protocol can be easily adapted to different traffic load situations. Furthermore, there is no infrastructure needed in such a distributed topology, which makes the network setup very flexible. However, in contention-based MAC schemes the system is faced with the exposed terminal and hidden terminal problem because the access decision is made locally by the node.
In general, contention-based MAC protocols are suitable for a network without central controller or with light traffic load, while schedule-based MAC protocols are appealing to centralized networks with busy traffic. Usually, contention-based schemes cannot provide Quality-of-Service (QoS) guarantee.
At the end of 2007 task group IEEE 802.15.6 known as Wireless Body Area Network (WBAN) was launched. IEEE 802.15.6 is intended to endow a future generation of short-range electronics, both in the body and on or around it, with a wireless communication standard for exchanging information. The initial intention with this new standard comes from the medical application purpose. However, given the considerable interest from industries, consumer electronics (CE) related applications are also expected to be supported by this new standard.
Medical and CE applications have greatly different characteristics in terms of traffic arrival, data rate, access delay and reliability requirements. In medical applications the traffic is typically periodic and of a data rate below 100 kbps with strict latency requirement. In CE applications the traffic can be streaming or burst-based with a data rate up to 10 Mbps. In the discussion of the IEEE task group, it is desirable that a unique MAC protocol be proposed to meet the requirements from both medical and CE applications. This makes MAC protocol design a new challenge in such a specific scenario.
Body area networks (BAN) comprise a set of mobile and compact sensors, either wearable or implanted into the human body. Both medical related applications and CE applications are to be implemented in BAN. Due to the medical applications to be supported, the design target of the MAC protocol is that the QoS be guaranteed for the medical application and that at the same time the energy be used in an efficient way. Generally speaking, there are three key parameters to evaluate the performance of different MAC schemes: throughput, latency and energy efficiency. For the two types of applications, there are different emphases on the performance evaluation. For the medical applications it is important to achieve better latency and energy consumption performance. For the CE applications, throughput and energy consumption are the two factors one is most concerned with.
Most of the existing research work on MAC layer design for sensor networks aims at reducing the energy consumption on account of the battery-powered sensor nodes. Sensor-MAC (S-MAC) (as known e.g. from the paper “An Energy-efficient MAC Protocol for Wireless Sensor Networks,” W. Ye et al., Proc. IEEE Infocom, pp. 1567-1576, New York, June 2002) is designed to coordinate sleeping among neighbor nodes in order to avoid idle listening and to extend network lifetime. However, this mechanism cannot cope properly with heavy traffic load. To solve these problems, timeout MAC (T-MAC) (see “An Adaptive Energy-efficient MAC Protocol for Wireless Sensor Networks”, T. van Dam et al., Sensys03, pp. 171-180, Los Angeles, Nov. 2003) is proposed as an extension to the S-MAC, which allows a node to go back to sleep when there is no traffic detected for a certain period of time. T-MAC still suffers from the overhead due to synchronization and increased latency. Hence, in the state-of-the-art there is no existing protocol that meets the new requirements of body area networks.
IEEE 802.15.4 is currently the major technology for regulating low-rate low-power wireless personal area networks (WPANs), in which the contention-based and schedule-based MAC schemes are used in a combined way, considering that WPAN is the most related scenario to the WBAN. In this standard the physical layer (PHY) and medium access control (MAC) layer are specified, based on which the ZigBee Alliance defines the upper layer protocols.
An IEEE 802.15.4 network can operate in either beacon-enabled or non-beacon-enabled mode. In the beacon-enabled mode, the network is operated in a synchronized way, while in the non-beacon-enabled mode the system works in an asynchronous way. The beacons are used to synchronize all devices in the network and to bound the superframes. Using a superframe structure a coordinator on a personal area network in IEEE802.15.4 can bound its channel time. One superframe is divided into 16 equally sized slots. Optionally, the superframe can have an active and an inactive portion. The coordinator may enter a low-power (sleep) mode during the active portion. In the active part the superframe is partitioned into three parts: a beacon, a contention access period (CAP) and a contention-free period (CFP). A WPAN may consist of multiple traffic types, including periodic data, intermittent data, and repetitive low-latency data. In the CAP part the nodes employ the CSMA with Collision Avoidance (CSMA-CA) mechanism to access the channel. The CFP part is divided into guaranteed time slots (GTSs) on a reservation based approach to accommodate the periodic traffic. A node can first send a GTS request on the CAP part using CSMA-CA and then the network coordination will decide on the GTS allocation. The IEEE802.15.4 superframe structure is shown in FIG. 1.
The CSMA-CA algorithm is used before the data transmission or before. MAC command frames are transmitted within the CAP. If periodic beacons are used in the PAN, the MAC sublayer employs the slotted version of the CSMA-CA algorithm for transmissions in the CAP of the superframe. Conversely, if periodic beacons are not used in the PAN or if a beacon cannot be located in a beacon-enabled PAN, the MAC sublayer transmits using the unslotted version of the CSMA-CA algorithm. In both cases the algorithm is implemented using units of time called backoff periods.
In the slotted CSMA-CA procedure the basic time slot unit is a backoff period, which is aligned with the superframe boundary. The access contention procedure is determined by three variables: NB, CW and BE. NB stands for the maximum backoff time allowed in one transmission attempt. CW is the contention window length, which is the number of backoff periods that the channel should be free of activity before a transmission can commence. The value of CW is defined to be 2 in the standard. Backoff exponent BE determines the maximum number of backoff periods a node should wait before attempting to access the channel.
The slotted CSMA-CA procedure works as follows. First the number of backoff periods is initialized to 0. BE is set to the minimum BE parameter MinBE. The node randomly selects a number within the interval of [0, 2BE−1] to initialize the backoff delay counter. On each slot boundary the backoff counter is decremented by one. When the counter reaches zero, it performs a clear channel assessment (CCA). If the carrier is detected to be idle, the node begins to transmit in the following slot. Otherwise, both the number of backoff periods and the variable BE are increased by 1, after which another transmission attempt begins. With BE upper bounded by the parameter MaxBE, the transmission attempt repeats until a successful transmission takes place or the number of backoff periods reaches NB. The slotted CSMA-CA procedure is presented in FIG. 2.
The CFP starts on a slot boundary immediately following the CAP and it shall terminate before the end of the active portion of the superframe. If any GTS has been allocated by the PAN coordinator, it is located within the CFP and occupies contiguous slots. The CFP therefore grows or shrinks depending on the total length of all of the combined GTSs. No transmissions within the CFP use a CSMA-CA mechanism to access the channel.
In the paper “AGA: Adaptive GTS Allocation with Low Latency and Fairness Considerations for IEEE 802.15.4”, (Y. Huang et al., IEEE ICC 2006, Vol. 9, pp. 3929-3934, June 2006) a priority-based scheme at the coordinator end is proposed for GTS allocation in IEEE 802.15.4. However, all the nodes still have to first compete on the CAP part to send the GTS requests to the coordinator.
In conclusion, when using existing access methods for at least two applications with substantially different characteristics, which requires guaranteed communication for at least one critical application and high data rate support for at least one other non-critical application, tuning of the parameters thereof can be done to give priority of one application over another. However, this does not result in a priority guarantee nor it is very efficient for cases where the traffic requirement of those applications are quite different.